Badly Rusted (& Crud Filled) Fuel Tank

DIY. Sluice it out as best you can then let it sit for 24/48hrs full of a weak phosphoric acid solution.

£20 for 5 litres..... Google it!

 

Don't know if this is helpful and might not be what you're looking for but...
I had probs with the tanks from my two 90's Hondas. Basically they were rusting. Particles were flaking off, clogging filters and buggering up carbs. I tried de-rusting, using a product mentioned in one of the monthly comics. I'm pretty sure I followed the instructions carefully but it didn't work. It was bloody difficult trying to dry out the tank at the end because of the convoluted baffle system inside. I had them sitting over an oil-filled rad for 3 days!
In the end I had the tanks lined. There are DIY kits out there but after my de-rusting experience I let the professionals do it. I went to biketankrepairs.co.uk. They're in Devon and the main man is Kevin Newton. So far everything has been fine with no trace of rust.

 

Just fill and forget, cheap pickling vinegar and top up with water to the brim,leave a few days....